[ Diff(x[1], t) = (1*k[1]*k[2] + 1*k[1]*k[4]*x[1]/(k[3] + x[1]) + (-1)*k[1]*k[5]*x[1]*x[2] + (-1)*k[1]*k[6]*x[1])/k[1],
  Diff(x[2], t) = (1*k[1]*k[7] + (-1)*k[1]*k[8]*x[1]*x[2] + (-1)*k[1]*k[9]*x[2])/k[1] ]